A study on the contact angles of a water droplet on smooth and rough solid surfaces

We investigated the wetting characteristics such as contact angle, wetting radius and topography of water droplets on smooth and random solid surfaces. Molecular dynamic simulation is employed to analyze the wetting behavior of water droplets on smooth and rough surfaces by considering different potential energy models of bond, angle, Lennard-Jones and Coulomb to calculate the interacting forces between water molecules. The Lennard-Jones potential energy model is adopted as an interaction model between water molecules and solid surface atoms. The randomly rough surface is generated by changing the standard deviation of roughness height from 1 Å to 3 Å with the fixed autocorrelation length. The size of water droplet considered is in the range from 2,000 to 5,000 molecules. The contact angles increase generally with increasing number of water molecules. For a hydrophobic surface whose characteristic energy is 0.1 kcal/mol, the contact angles depend rarely on the standard deviation of the roughness height. However, when the surface energy is 0.5 and 1.0 kcal/mol, the contact angles depend on both the roughness height of surfaces and droplet size.     

Flow around a freely falling square shape particle in a channel using direct-forcing fictitious domain method

This paper presents a numerical investigation of the flow characteristics around a freely falling square shape particle in a two-dimensional channel. The FSI (fluid-solid interaction) has been realized by using the direct-forcing/fictitious domain (DF/FD) method. In order to identify the effect of fluid property on the flow characteristics and solid motion by FSI, a wide range of the fluid viscosity has been considered, which introduces various Reynolds numbers to this study. In addition, the off-centered distance of the square particle has been imposed to research the effect of the initial position. The centered particle is freely falling without rotation and transverse motion. However, the motion of the off-centered particle is significantly depended on the Reynolds number, so it is classified into the four regimes by the flow and moving characteristics of the particle. Quantitative information about the drag coefficient on the square particle is highlighted.     

Heat transfer and pressure drop amidst frost layer presence for the full geometry of fin-tube heat exchanger

The present study numerically solves the flow and thermal fields in the full geometry of heat exchanger modeling with frost layer presence on the heat exchanger surface. The effects of air inlet velocity, air inlet temperature, frost layer thickness, fin pitch, fin thickness, and heat exchanger shape on the thermo-hydraulic performance of a fin-tube heat exchanger are investigated. Heat transfer rate rises with increasing air inlet velocity and temperature, and decreasing frost layer thickness and fin pitch. Pressure drop rises with increasing air inlet velocity and frost layer thickness, and decreasing fin pitch. The effect of fin thickness on heat transfer and pressure drop is negligible. Based on the present results, we derived the correlations, which express pressure drop and temperature difference between air inlet and outlet as a function of air inlet velocity and temperature, as well as frost layer thickness.     

Three-dimensional fin-tube expansion process to achieve high heat transfer efficiency in heat exchangers

A novel manufacturing process for expanding the tubes of fin-tube type heat exchangers using a three-dimensional (3D) spiral expanding ball fabricated via metal additive manufacturing was proposed for the manufacture of highly efficient heat exchangers. To improve the heat transfer efficiency of fin-tube type heat exchangers, fine grooves are generally formed inside a tube to increase the heat transfer area. However, the height of a groove is commonly reduced when a tube is expanded for tightening with fins. To address this issue, a 3D expanding ball with spiral grooves was first developed and used in the expansion process. In conventional tube expansion, the height reduction of grooves is approximately 10.3 %. However, we demonstrated that it was dramatically improved, reaching approximately 1.7 %, when the proposed process with a 3D expanding ball was applied. We believe that this approach can be used in practical industries to manufacture highly efficient fin-tube heat exchangers.